1. Field of the Invention
The present invention pertains to a semiconductor device comprising an ultra-high frequency transistor that works as a low-noise amplifier and is encapsulated in a package with an internal cavity. This ultra-high frequency transistor is matched with a standardized external impedance of 50 ohms at input and output. It can therefore be used in a wide range of frequencies without any external impedance-matching circuit: it is pre-matched inside the package: the pre-matching at the terminals of the chip of semiconducting material gives more gain and greater band width than an external matching at the terminals of the package. This pre-matching also gives lower noise factor.
2. Description of the Prior Art
The making of ultra-high frequency amplifiers, in a range of 6 GHz to 100 GHz for example, raises two types of problems. Firstly, in the current state of the art, it is not possible to make a hermetically sealed package that works at more than 25 GHz for low -noise amplifiers. Yet, to be mounted on a satellite, a directional radio link system or military equipment, it is necessary for ultra-high frequency amplifiers to be in a package. Each frequency band has a corresponding package that is matched with an external impedance.
Furthermore, to reduce noise and increase the gain of an ultra-high frequency transistor chip, the dimensions of the gate must be reduced. However, in this case, the optimum frequency of the transistor changes: it goes from the band 6-18 GHz, for example, to the band 18-40 GHz. A chip modified in this way can no longer be used in a package mounted between two 50-ohm lines at 12 GHz.
3. Summary of the Invention
To remove these disadvantages, the present invention proposes the pre-matching of a chip of a low-noise ultra-high frequency transistor directly inside the encapsulating package, the pre-matching circuit being mounted between the metallic terminals of the transistor chip and the internal ends of the external connections of the package. Since the reduction in the dimensions of the gate, made in order to reduce noise, modifies the impedance of the transistor and brings the said impedance closer to that of the open circuit, this impedance modification is corrected by inserting a pre-matching circuit inside the package, the said pre-matching circuit comprising at least one choke between the gate terminal or the drain terminal and the corresponding external connection.
An object of the invention, therefore, is to obtain, through internal pre-matching in the package, an ultra-high frequency transistor with less and more gain, mounted between two 50-ohm lines. Since it is matched, the low-noise amplifier also has more gain and a greater band width.
Another object of the present invention is to be able to encapsulate a low-noise transistor chip, working at 40 GHz for example, in a package that can be used up to 25 GHz, since it is not possible at present to make hermetically sealed packages that work at frequencies of beyond 25 GHz.
According to an improvement of the invention, a second matching circuit is added inside the package in order to approach the minimum noise factor, the said second matching circuit being connected to the internal end of an external connection. This circuit, comprising a choke which is series-connected with a capacitor, improves the gain if it is connected to the drain and decreases the noise if it is connected to the gate.
More precisely, the invention comprises a low-noise, ultra-high frequency semiconductor device comprising a transistor mounted in a package, the said package having an internal cavity and at least two external connections, the transistor chip being soldered to a metallic or metallized base inside the package and being provided with contact terminals on its gate and drain electrodes, the said semiconductor device being pre-matched in impedance by at least one choke which is series-mounted between either the gate electrode or the drain electrode of the transistor and its corresponding external connection, the said choke, inside the package, comprising a metallic wire or microstrip which is fixed between a terminal on the transistor chip and the internal end of an external connection, and which forms a hairpin that has its two strands substantially parallel to each other.